The present invention relates to a pressure-sensitive ignition plug for use in internal combustion engine for automobiles and, more particularly, to an improvement in the pressure sensing unit and signal transmission system of a pressure sensitive ignition plug.
It is a current tendency to use ignition plugs having a pressure sensing unit sensitive to the internal pressure of the cylinder of engine, in order to control the knocking taking place in the engine. This type of ignition plug is generally referred to as "pressure-sensitive ignition plug." The use of the pressure-sensitive ignition plug is advantageous in that it can eliminate the necessity for a specific bore for detecting the internal pressure other than the bore for fitting the ignition plug.
The pressure sensing unit usually incorporates a piezoelectric element. An example of the pressure-sensitive ignition plug using a pressure sensitive unit incorporating a piezoelectric element is disclosed in the specification of the U.S. Pat. No. 3,020,763. The piezoelectric element exhibits a high output voltage of an order of several volts when the circuit connected to the output terminals thereof is opened. However, since the piezoelectric element has an extremely high impedance, the level of the output signal from the piezoelectric element is largely reduced due to connection of load including lead lines, and noises are introduced into the signal line to make it difficult to discriminate the output signal.
In order to obviate this problem of the prior art, it has been proposed to use a strain gauge or a variable capacitance type element in the pressure sensing unit, in place of the piezoelectric element. These elements, however, can provide only weak output signals, so that it is quite difficult to distinctively and sufficiently amplify only the output signal particularly in the area around the cylinder where the level of the noise is specifically high. Namely, it is difficult to obtain a high S/N ratio with these pressure-sensitive elements.